Triple quadrupole mass spectrometry is a well established analytical technique for the targeted analysis of complex mixtures. In a triple quadrupole mass spectrometer, ions are generated from an ion source and injected into a first quadrupole analyzer. Here, a narrow mass range (m/z)) is selected and this narrow mass range enters a second stage which comprises a gas filled collision cell. Fragment ions generated by collisions with gas enter a second quadrupole analyzer where a particular fragment is selected for detection.
The triple quadrupole technique permits the isolation of precursor and corresponding fragment ions of interest, thus providing a robust quantitative method for target analysis, in the case that the targets for analysis are known but are present at very low levels compared to other analytes.
A drawback of this analytical method is that only a narrow window of m/z is isolated in the first stage, with all other m/z being lost on the quadrupole rods. This wasteful operation hinders rapid quantitation analysis where multiple target compounds need to be analyzed within a limited time. The quadrupoles must in each case be set to accept a different range of m/z, and effective duty cycles are quite low (perhaps 0.1%-10%, depending upon the number of targets).
An alternative to the traditional triple quadrupole mass spectrometer involves simultaneous acquisition of all fragments from all precursors in one high resolution, high mass accuracy spectrum. Once that single spectrum has been obtained, it can be searched to try to identify ions of a target m/z. Analyzers having sufficient resolution and mass accuracy to allow implementation of this effect include the Orbitrap™ electrostatic trap analyzer and the time-of-flight (TOF) analyzer. However, even with such instruments (resolving power >50,000 to 100,000 and mass accuracy below 2 ppm or even better), the extremely large ranges of concentrations in modern targeted analysis experiments mean that existing so-called “all mass” analyzers cannot rival the triple quadrupole device in terms of linearity, dynamic range and detection limits for a specific m/z of interest. For TOF analyzers, the limitations result from low transmission and detection electronics constraints. For the Orbitrap™, the difficulty is primarily the limited charge capacity of any external trapping device.
One way of improving throughput of mass analysis is to carry out MS/MS where the ion beam is split into packets in accordance with the packets' m/z. A first packet is then fragmented without loss of another packet, or in parallel with another packet. The splitting of the ion beam into packets can be achieved by the use of a scanning device which stores ions of a broad mass range. Suitable devices for implementing this scanning are a 3D ion trap, such as is disclosed for example in WO-A-2003/103,010, a linear trap having radial ejection as is described in U.S. Pat. No. 7,157,698, a pulsed ion mobility spectrometer (see, for example, WO-A-00/70335 or US-A-2003/0213900), a slowed down linear trap (see WO-A-2004/085,992) or a multi-reflection time of flight mass spectrometer such as described in WO-A-2004/008,481.
In each case, the first stage of mass analysis is followed by fast fragmentation in a collision cell for example (preferably, a collision cell having an axial gradient), or by a pulsed laser. The resulting fragments are analyzed using, for example, another TOF mass spectrometer, but on a much faster time scale than the scanning duration (known as “nested times”). The performance is still however compromised because only a very limited time is allocated for each scan (typically, 10-20 μs).
These approaches of so-called “2-Dimensional MS” seem to provide throughput without compromising sensitivity, unlike the more traditional multi-channel MS/MS arrangement wherein a number of parallel mass analyzers (typically ion traps) are used to select one precursor each and then scan out the fragments from that precursor to an individual detector such as the ion trap array disclosed in U.S. Pat. No. 5,206,506 or the multiple traps disclosed in US-A-2003/089,846.
All known 2-Dimensional MS techniques suffer however from the relatively low resolution of precursor selection (not better that unit resolution) and the relatively low resolving power of fragment analysis (not more than a few thousands). Also, these known 2-Dimensional MS techniques are based on the use of trapping devices to provide a high duty cycle, and the cycle time is defined by the cycle time of the slowest analyzer. Modern ion sources can produce ion currents of the order of hundreds of picoAmps, that is, in excess of 109 elementary charges per second. Thus, if the full cycle of scanning through the entire mass range of interest is 5 ms, then such trapping devices should in principle be able to accumulate up to 5 million elementary charges and still allow efficient precursor selection.
WO-A-2008/059246 describes an arrangement that permits high performance simultaneous isolation of multiple ion species, either for subsequent detection or fragmentation. In the disclosed arrangement, ions are injected into a multi-reflection electrostatic trap which reflects ions back and forth along an axis. Ions of species of interest are isolated by appropriate control of an electrostatic gate which diverts ions in accordance with their period of oscillation within the trap, along first or second ion paths respectively.
Against this background, the present invention provides, in a first aspect, a method of tandem mass spectrometry in accordance with claim 1. The invention also extends to a tandem mass spectrometer in accordance with claim 21.
The invention is based upon the realisation that targeted analysis does not require all MS/MS spectra to be acquired independently. The instrument merely needs to deliver separated and detectable peaks for the ion species of interest. These separated precursors may have their populations mixed together again and then acquired in a single high resolution spectrum. This so called parallel reaction monitoring (PRM) allows quantification of multiple low intensity analytes in parallel, thus greatly increasing the detection limits over triple quadrupoles in massive targeting experiments.
The ions selected at the ion gate for onward transmission to the ion guide may remain in an unfragmented state upon arrival at the ion guide, and downstream of that as they are analyzed in the high resolution mass analyser. This mode greatly extends the capabilities of the above described “all-mass analysis” technique, by opening up the possibility of storing m/z of different intensities by using different duty cycles. In that way, both the unfragmented and fragmented spectra are obtained with a range of intensities that has been reduced by 1-3 orders of magnitude. For example, low-intensity peaks might be transmitted to the high resolution mass analyser after every injection, whilst high-intensity peaks might only be transmitted during 0.5-1% of all injections. The various relatively small mass range spectra obtained (each typically having its own particular attenuation scheme) can optionally then be stitched together (for example by using the technique described in WO-A-2005/093783). With a final spectrum corrected for these differences in transmissions, such “spectrum stitching” allows for a significant extension to the dynamic range of analysis.
In addition, the technique employed provides sufficient time to fragment ions, and in particular provides sufficient time to employ such recently developed “slow” techniques as Electron Transfer Dissociation (ETD) or infrared multiphoton dissociation (IRMPD).
Thus in accordance with some preferred embodiments of the present invention, some or all of the precursor ions allowed to pass through the ion gate may be fragmented downstream thereof. In one preferred embodiment, the ion guide comprises a fragmentation cell and an ion trap (which may optionally be a second ion trap) downstream of that fragmentation cell. Precursor ions of interested are then selected by the ion gate, and passed to the fragmentation cell where some or all of the precursor ions are fragmented. The fragment ions (and any remaining precursor ions) are then analysed by the high resolution mass analyser. Most preferably, the fragment ions are stored in the (second) ion trap so that, for example, particular low abundance species can be augmented in that (second) ion trap through multiple cycles of the technique, prior to high resolution mass analysis. In additional or alternative embodiments, augmentation of precursor ions may take place as well or instead, in the ion accumulation means, either by using a fragmentation cell but operating it in a low energy mode so that ions are not fragmented, and/or by bypassing the fragmentation cell (or omitting it entirely) and employing a second ion trap.
Thus, multiple m/z ranges can be selected (rather than 1, as in quadrupole mass filters) from a wide mass range of precursors. Each selected precursor species can be fragmented—optionally at a respective optimal energy—and the fragments can then be combined in a single broad spectrum fragment population. This single fragment population can then be analyzed in a high resolution mass analyzer such as a TOF, an orbital electrostatic trap such as the Orbitrap™, or FT-ICR mass spectrometer. Thus a method and apparatus is proposed that addresses on the one hand the limited space—charge capacity of trapping analyzers, and the limited dynamic range of TOFs on the other, by selecting a limited but nevertheless plural number of ion species of analytical interest for fragmentation and subsequent parallel analysis. For example, between 10 and 100 precursor species could be analyzed together in this technique.
The present invention may provide for a method of tandem mass spectrometry, comprising the steps of a) generating precursor ions in an ion source; b) trapping the precursor ions in an ion trap; c) ejecting the precursor ions from the ion trap towards an ion guide, via an ion gate, so that the precursor ions arrive at the said ion gate only once on their passage to the said ion guide, the precursor ions arriving as a temporally separated plurality of ion packets each containing ions of a respective one of a plurality of different ion species; d) controlling the ion gate so as to sequentially select from the plurality of ion packets arriving at the ion gate, a subset of a plurality of ion packets deriving from a subset of precursor ion species of interest; e) mixing the selected subset of a plurality of ion packets in the ion guide; and f) analyzing the resulting ion population derived from the mixed selected subset of ion packets in a high resolution mass analyzer.
It may also provide for a tandem mass spectrometer comprising an ion source for generating precursor ions; an ion trap arranged downstream of the ion source, for trapping precursor ions from the ion source; a single pass ion gate, arranged in a path of precursor ions ejected from the ion trap towards a downstream ion guide, the precursor ions arriving at the said ion gate as a plurality of temporally separated ion packets each containing ions of a respective one of a plurality of different ion species; an ion gate controller configured to control the single pass ion gate so as to permit passage of only a subset of ion packets containing a respective subset of a plurality of precursor ion species of interest; wherein the ion guide is configured to receive precursor ions that are permitted to pass through the single pass ion gate; the tandem mass spectrometer further comprising: a high resolution mass analyzer arranged to analyze the ions or their fragments.